The present invention relates to the field of semiconductor processing, and more particularly, to the formation of metallic gate electrodes using the replacement gate process technique.
In the integrated circuit (IC) industry, metal-oxide-semiconductor (MOS) transistors have typically been formed utilizing polysilicon gate electrodes. Polysilicon material has been preferred for use as an MOS gate electrode due to its thermal resistive properties (i.e., polysilicon can better withstand subsequent high temperature processing). Polysilicon""s robustness during high temperature processing allows polysilicon to be annealed at high temperatures along with source and drain regions. Furthermore, polysilicon""s ability to block the ion implantation of doped atoms into a channel region is advantageous. Due to the ion implantation blocking potential of polysilicon, polysilicon allows for the easy formation of self-aligned source and drain structures after gate patterning is completed.
However, polysilicon gate electrodes have certain disadvantages. For example, polysilicon gate electrodes are formed from semiconductor materials that suffer from higher resistivities than most metal materials. Therefore, polysilicon gate electrodes may operate at much slower speeds than gates made of metallic materials. To partially compensate for this higher resistance, polysilicon materials often require extensive and expensive silicide processing in order to increase their speed of operation to acceptable levels.
A need exists in the industry for a metal gate device which can replace a polysilicon gate device. However, metal gates can not withstand the higher temperatures and oxidation ambients which can be withstood by conventional polysilicon gate electrodes. In efforts to avoid some of the concerns with polysilicon gate electrodes, a replacement damascene metal gate process has been created. A damascene gate process uses a disposable gate, which is formed with a source, drain, spacer, etch stops and anti-reflective coatings as in conventional processing. The disposable gate and dielectrics are etched away, exposing an original gate oxide. The disposable polysilicon gate is then replaced by a metal gate to achieve the lower resistivity provided by the metal material.
A design consideration in semiconductor technology is that of the work function, which is the amount of energy required to excite electrons across a threshold. Polysilicon gates on silicon substrates provide a work function that allows the gates to be adequately controlled. The use of metal, however, as the gate material on a silicon substrate undesirably changes the work function in comparison to polysilicon gates. This reduces the controllability of the gate.
There is a need for a semiconductor structure and arrangement for making the same in which the gate is made of a metal, but the work function is substantially the same as a semiconductor structure which contains a polysilicon gate.
This and other needs are met by the embodiments of the present invention which provide a semiconductor structure comprising a substrate, active regions in the substrate, and a gate structure on the substrate. This gate structure includes a high dielectric constant (high k) gate dielectric on the substrate, a chemical vapor deposited (CVD) layer of amorphous silicon on the high k gate dielectric, a barrier on the CVD amorphous silicon layer, and a metal on the barrier. The barrier comprises a material that prevents interaction between the metal and the CVD amorphous silicon layer.
By providing a semiconductor structure having a gate structure with a CVD layer of amorphous silicon and a metal on the CVD amorphous silicon layer, the advantages of a metal gate, including that of lower resistivity, is achieved without compromising the work function of the gate structure. Hence, the CVD amorphous silicon layer causes the work function of the metal gate to appear like a standard gate. The barrier provided between the CVD amorphous silicon layer and the metal prevents interaction, such as silicidation, from occurring during further processing, since the CVD amorphous silicon layer is not undesirably silicidized, the work function of the gate is preserved. Hence, the barrier allows higher temperatures to be employed in continued processing of the device, without concern that the work function will be compromised by unintentional formation of silicide within the gate electrode.
The earlier stated needs are also met by embodiments of the present invention that provide a method of forming a semiconductor structure, comprising the steps of forming a precursor having a substrate with active regions separated by a channel, and a temporary gate over the channel and between dielectric structures. The temporary gate is removed to form a recess with a bottom and sidewalls between the dielectric structures. Amorphous silicon is deposited in the recess by chemical vapor deposition. A barrier layer is deposited over the amorphous silicon. The metal is then deposited in the recess on the barrier layer. The barrier layer prevents interaction of the metal with the amorphous silicon.
The formation of a semiconductor structure in accordance with the present invention is advantageous in that high-temperature processes may be performed prior to the deposition of the metal gate. Also, the formation of source and drain electrodes self-aligned to the subsequently formed metal gate is possible. The formation of the metal gate in this replacement gate process, however, allows the metal gate to be formed after the implantation of the dopant atoms. By depositing amorphous silicon in the recess by chemical vapor deposition prior to the depositing of the metal in the recess on the amorphous silicon, the work function will be same as if the gate were made of polysilicon instead of metal. This provides increased control of the gate and avoids leakage. The work function is preserved by the use of a barrier layer that prevents interaction between the CVD amorphous silicon layer and the metal layer, even at higher temperatures during further processing.
The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.